Wavelength multiplexing optical transmission systems employ an optical amplifier that collectively amplifies signal lights to compensate attenuations of signal lights through an optical fiber. An example known optical amplifier is an optical amplifier that utilizes erbium-doped optical fiber. This optical amplifier simultaneously inputs a signal light and an excitation light to the erbium-doped optical fiber, thereby amplifying the signal light.
According to optical amplifiers, when the power of input light changes, the wavelength dependency of the gain also changes. Hence, if this type of optical amplifier is applied to a wavelength multiplexing optical transmission system, the gains of signal lights change in accordance with the number of multiplexed signal lights. Therefore, various technologies of maintaining the gains of respective signal lights constant have been proposed (see, for example, Patent Literature 1).
The device disclosed in Patent Literature 1 has two optical amplifiers connected in series, and causes the first-stage optical amplifier to amplify respective input signals so as to maintain constant the ratio of the level of the input light to that of the output light. Next, the second-stage optical amplifier amplifies the respective signal lights so as to maintain constant the level of the input light to that of the output light, and compensates the wavelength dependency of the gain at the first-stage optical amplifier.